The present invention relates generally to a converting process and more particularly to a method for controlling the surface charge on a printed substrate used in a converting process. Specifically, the invention is disclosed with respect to polymer coated substrates printed with EB (electron beam) cured inks. The printed surface may either be an uncoated or clay coated surface.
The high speed of polymerization offered by electron beam curing provides a new tool for the manufacture of unique products. For example, a wet electron beam cured coating, which is typically applied from a roll surface or from a gravure cylinder, has little time to soak into or penetrate the substrate prior to cure if the dwell time-to-viscosity relationship is well controlled. When energetic electrons from the electron beam are absorbed by the coating, they produce a polymerization reaction between the multifunctional monomer and the unsaturated prepolymer in the coating in a period of tens of milliseconds. Thus, a wet coating will be converted to a tack free film in a matter of only a few milliseconds after passing through the electron beam. This feature has led to a number of EB applications on paper, particularly printing processes. However surface charge problems have arisen in some cases where the EB printing is applied to paper or paperboard containing a polymer coating.
Charge accumulation in polymers due to EB irradiation is not new. These charges are caused by electrons becoming trapped in the polymer. For instance, in an EB printing operation on paperboard, the primary electrons impinge on the inked surface of the paperboard. However, this surface absorbs only about 40% of the electrons to effect curing. The remaining electrons penetrate the paperboard surface where they produce a large number of secondary electrons, caused by primary electrons ionizing molecules within the paperboard substrate. Some of these secondary electrons along with as many as 10% of the primary electrons can reach polymer coatings applied to the opposite surface of the paperboard substrate. These electrons become trapped in the polymer layer to produce an overall positive surface charge on the substrate. The amount of charge trapped depends in large part upon the type of polymer coating applied to the substrate and the moisture content of the paper or paperboard. Polyethylene terephthalate (PET) is twenty times more efficient at trapping a charge than polyethylene because of the presence of a large number of unsaturated bonds in the PET. Likewise, the lower the moisture content of the paper or paperboard, the greater the accumulated charge. In any event the accumulation of too great of a surface charge on the substrate plays havoc with subsequent converting operations using the substrate. Devices and methods for removing or applying an electrical charge to paper during converting processes are known as taught in U.S. Pat. Nos. 2,483,542 and 3,670,203. However, such devices have proven to be ineffective at removing the accumulated charge on polymer coated substrates. Accordingly, the present invention was developed to control the charge on a polymer coated paper or paperboard substrate subjected to EB irradiation by removing or adding a charge as desired.